A large number of different temperature measuring methods exists. For example, temperatures can be measured by means of thermal expansion (liquid thermometers, bimetallic strain gauges), the temperature dependence of the electrical resistance (resistance thermometers) or the thermoelectric voltage at the interface of different metals (thermocouple). Other methods are based on electromagnetic irradiation (infrared radiometers, pyrometers). These measuring methods are very suitable for determining the temperature in the case of solids and liquids.
Apart from gas expansion measurements performed in the gas thermometer, gas temperatures are generally indirectly measured, by bringing the gas into contact with a solid or liquid thermometer. Due to the poor thermal conductivity of gases and the low thermal energy transfer from the gas to the thermometer, such temperature measurements are relatively slow.
On the basis of the ideal gas law, the temperature of an enclosed gas can be determined via its pressure. The pressure fluctuations occurring in the case of a periodic gas temperature change, can be determined in a highly sensitive manner by means of a microphone. However, due to the aforementioned low thermal conductivity of gases, the temperature changes which occur are generally very small, so that it would be necessary to operate the microphone at very low frequencies in the subhertz range. However, microphones are not very suitable for such frequencies or even for static measurements. It is therefore of interest to seek a process permitting a direct and quasi-static measurement of the gas temperature.
Numerous uses are conceivable where it is desired to measure small temperature changes in a gas. For example, in optical gas spectroscopy, it is of interest to investigate the interaction of the radiation field with a gas. On the basis of a periodic light and in particular infrared irradiation, periodic temperature fluctuations occur in the gas.
Different devices exist for measuring the light power absorbed in a gas. The greatest significance is attached to solid radiation sensors based on the photoelectric effect. Using such sensors, use is generally made of the so-called extinction method. The absorption of the material being investigated is determined as a result of the comparison of two beams, one beam passing through the material and the other passing unimpeded to the detector. However, as opposed to this indirect method, preference is often given to a direct method, which consists of directly measuring the light absorption-caused signal. As stated, it has proved very satisfactory in this connection to measure pressure fluctuations building up in a closed cell on absorbing intensity-modulated light. This so-called photoacoustic effect is characterized by a very high sensitivity. As the measurement takes place with the aid of a microphone, it is not possible to prevent acoustic interference to the measurement, and in many cases this is not acceptable.
Another possible use of a direct gas temperature measuring method is calorimetry.
Calorimetric research is carried out in the standard calorimeter in an aqueous ambient. The measuring material is either placed directly in the water, or it is immersed in the calorimeter water in a closed vessel. The latter is located in a water tank thermally separated from the environment. The calorimetric test is based on following the temperature of this calorimeter water, to which the measuring material gives off its heat which is liberated, for example, by a chemical reaction. It must be borne in mind that the temperature distribution within the calorimeter must be as homogeneous as possible and must also coincide with that of the material being analyzed. For this purpose it is advantageous to use a stirrer.
Water is suitable as the heat transfer medium from the material to the thermometer, on the one hand due to its good thermal conductivity and on the other due to the rapid attainment of a homogeneous temperature distribution by stirring. However, in certain cases it is difficult, or even impossible, to place the reactor in the water bath of a calorimeter. Therefore calorimetric measurements are often carried out in a gaseous environment, e.g., in air, or even in vacuum, the material being placed on a substrate in the form of a disk or a crucible provided with temperature sensors. Reference is made to the differential calorimeter.
Two problems occur in the case of the gas calorimeter. On the one hand it is necessary to ensure good thermal contact between the material and the substrate and on the other hand the calorimeter chamber or container must be thermally well insulated from the environment.
The construction of a gas calorimeter, such as is e.g. used for determining the melting and reaction heats, will be described on the basis of the example of a differential calorimeter. Two crucibles are placed on a disk or plate in a cavity or container. One contains the reactive, solid or liquid material and the other an inert reference material. A thermocouple is in thermal contact with the two crucibles. In order to be able to operate the calorimeter at a high constant temperature or to be able to have a linear temperature gradient (differential scanning calorimeter), the calorimeter cavity is heatable and is optionally surrounded by a thermal insulation layer for shielding external heat sources.
If a thermally initiated reaction takes place in the material being investigated, e.g. during the heating process, then a thermoelectric voltage occurs in the thermocouple at the material contact point with respect to the reference material and this gives information on the relative heating of the material being analyzed.
It is of interest in integrated optics to determine the optical losses which are caused by the light absorption in the waveguide. Such an investigation can never be carried out in an aqueous environment, because the refractive index of water modifies the light guidance and coupling conditions and would consequently falsify the measurement. It is also virtually impossible to operate the optoelectronic element in the calorimeter container of a conventional gas calorimeter, because within the same there is no space for the means for coupling in and out light. Thus, the problem arises of carrying out a calorimetric measurement in air, in which the material is neither in good thermal contact with a substrate, nor can it be thermally adequately separated from the environment. One possibility is an indirect measurement of the enthalpy of the material over the heat loss to the environment.
Such a method is used for determining the optical losses in glass. It involves placing a glass rod in the optical resonator of a high-performance laser (P=115 W) on thin filaments and determining the stationary temperature rise .DELTA. T caused by the light passage, as well as the cooling time .tau. after switching off the laser. (T.C. Rich and D.A. Pinnow, Appl. Phys. Lett. 20, 264 (1972)). A fine thermocouple fitted to the glass specimen surface is used for measuring the temperature thereof. The light power .alpha. absorbed in the glass rod with a radius r and a thermal capacity C is calculated as: EQU .alpha.=C(.pi.r.sup.2) .DELTA.T/.tau.P (1)
Typically in the case of a Suprasil glass rod in the optical resonator of a 115 W laser, there was a temperature rise of .DELTA.T=0.56.degree. C. and on cooling a time constant .tau.=75s.
As a result of the process for preventing undesired heat removal, very thin thermocouple wires were used because the heat conduction of metals is higher by approximately 4 orders of magnitude than that of air. Problems are also caused by the thermal contact between the thermally poorly conducting glass rod and the thermocouple measuring point. Generally considerable effort and expenditure is required for the thermal contacting of fine specimens, e.g. if the thermocouple must be evaporated on for ensuring a good thermal contact. Furthermore, in certain cases of very fine optoelectronic components, a metallic thermal connection cannot be achieved, if e.g. the optical characteristics are disturbed by the temperature sensor.
These examples from optical spectroscopy and calorimetry are intended to show that there is an interest in directly measuring the temperature of gases.